The objective of the proposed study is to evaluate the pineal hormone, melatonin, as a potential biomarker of occupational exposure to magnetic fields. Although the epidemiologic data are inconsistent, occupational and residential exposure to magnetic fields appear to increase the risk for leukemia and brain cancer. However, the mechanism for induction of human health effects by EMF is unclear. Melatonin suppression is a biologically plausible mechanism which needs to be evaluated in human populations. Therefore, this study is designed to test the hypothesis that workers exposed to magnetic fields experience a decrease in melatonin production. The hypothesis will be tested by measuring magnetic field and light exposures in a group of electric utility workers with elevated EMF exposures and in a comparison group of workers. Magnetic fields will be measured on a 24 hour basis using individual data logging meters. Melatonin production will be evaluated by measuring urinary concentrations of its major metabolite, 6-hydroxymelatonin sulfate. If there is an association between EMF exposure and melatonin production in humans, urinary 6-hydroxymelatonin may be an important biomarker for evaluating the biological response of workers to EMF exposures. The specific aims of the project are: 1) to assess 24 hours exposures of electric utility workers to magnetic fields and light; 2) to characterize the day and night time production of melatonin in the same population of workers by measuring 6-OHMS; 3) to replicate preliminary data indicating that magnetic field exposure is associated with a reduction in 6-OHMS excretion; 4) to evaluate the individual differences in 6-OHMS excretion in response to magnetic field exposure during work, non-work and sleep periods; 5) to determine which characteristics of magnetic field exposure are most strongly associated with reduced 6-OHMS excretion and 6) to determine the characteristics of melatonin suppression in response to magnetic field exposure over a one year period.